Stereoscopy was introduced to radiology in 1898, and by 1930, most radiographs were taken stereoscopically. The technique used principally in early methods was to place a photographic plate behind the patient, take an exposure, replace the plate, shift the x-ray tube laterally, and take a second exposure. The developed films were then viewed in a stereoscopic viewer apparatus. The additional expense and exposure, with the discovery that overexposure to x-rays is harmful, led to the decline of stereoscopic x-ray techniques.
In order to reduce patient exposure by x-rays, intensifying and fluoroscopic screens were developed. Intensifying screens are fluorescent screens which emit light when the phosphors thereof are excited by x-rays. The light emitted exposes a photographic emulsion. In fluoroscopy, the fluorescent screen is viewed directly. Patient exposure is reduced by these methods several hundredfold.
One of the problems with early fluoroscopy was that the fluorescent screens were too dim for daylight (photopic) vision which is sharper than night (scotopic) vision. To overcome this problem, x-ray image intensifiers were developed. In an image intensifier, an input fluorescent screen absorbs x-ray photons and converts their energy into light photons. The light photons strike a photocathode causing it to emit photoelectrons which are drawn away by the high potential of an accelerating anode. An electrostatic lens focuses the electrons onto an output fluorescent screen which emits light which is viewed by the observer. Image intensifiers are often coupled with motion picture, television, or spot film cameras for recording the output fluorescent images.